The properties of nerve and muscle cells depend upon the organization of the surface membranes into structurally and functionally distinct domains whose lifetimes are longer than that of their constituents. The postsynaptic membrane at the neuromuscular junction is an example of such a domain. Its principal biochemical feature is the presence of a high concentration of acetylcholine receptors (AChRs), a protein whose molecular structure, orientation in the membrane and immunological properties are well-known. We propose to use the AChR as a molecular marker to study the synthesis and maintenance of the specialized postsynaptic muscle membrane. We postulate that assembly and maintenance of the membrane involve processes of direct local insertion and retrieval and that these are mediated via coated vesicles, whose role in intracellular membrane transport and recycling is well established. Our plan is to identify AChR-containing coated vesicles in developing muscles in vivo and in vitro and to develop immunological procedures for their purification. We shall compare their physical properties and their components to those of the bulk population. We will also determine if the AChR-rich coated vesicles contain components of the synaptic basal lamina, such as the 16S form of acetylcholinesterase (AChE). We will examine the expression of AChR-rich coated vesicles and of the total coated vesicle population during the sequential stages of development of the neuromuscular junction, in vivo and we will study in vitro their regulation by nerve and by muscle activity.